An in-mold expanded molded product made of pre-expanded particles is provided by expanding resin particles (material particles) as a material using a pre-expanding device, a decompressing expansion device, or the like to manufacture pre-expanded particles having predetermined bulk density and filling a mold of an in-mold molding machine with the pre-expanded particles for in-mold expansion molding. The in-mold expanded molded product is used for diversified shapes and applications (cushioning materials, heat insulating materials, car interior members, core materials for car bumpers, cushioning packaging materials, returnable boxes, and the like).
In particular, as a method for manufacturing the pre-expanded particles having a high expansion ratio, a two-stage expansion method has been known in addition to a method for providing the pre-expanded particles having the high expansion ratio by the normal pre-expanding device, the normal decompressing expansion device, or the like. In the two-stage expansion method, pre-expanded particles having a low expansion ratio are once obtained, and then, the pre-expanded particles are further expanded using a second-stage expanding machine so as to obtain pre-expanded particles (second-stage expanded particles) having a higher expansion ratio.
In this case, it is important to make bulk density of the second-stage expanded particles constant. The constant bulk density enables an amount of the second-stage expanded particles that are supplied to a molding machine to be stable, thereby stabilizing quality of a molded product.
In general, as the method for manufacturing the second-stage expanded particles, for example, a batch-type method has been known. In the batch-type method, pre-expanded particles (first-stage expanded particles) to which an internal pressure of equal to or lower than 0.50 MPa has been previously applied are put into a second-stage expanding machine and are expanded to have predetermined bulk density with a heating medium such as steam. In this case, as a method for adjusting the bulk density of the second-stage expanded particles, the following method has been employed. That is, the second-stage expanded particles that have been second-stage expanded are collected, and the bulk density thereof is measured. Then, operating conditions (for example, internal pressure of the pre-expanded particles, heating steam pressure at the time of the second-stage expansion, and the like) of the second-stage expanding machine for a subsequent batch are required to be changed based on difference between the measured bulk density and a target value. The bulk density of the second-stage expanded particles is frequently adjusted by changing the operating conditions of the second-stage expanding machine in this manner in order to make the quality of the molded product stable.
As for the second-stage expanded particles having such low expansion ratio that volumes of the second-stage expanded particles themselves do not contract immediately after the second-stage expansion, a bulk-density measuring device for the second-stage expanded particles and a measuring method therefor have been disclosed (see, Patent Document 1 and Patent Document 2). With the bulk-density measuring device and the measuring method disclosed in them, the second-stage expanded particles are collected into a vessel of a constant volume (V) under atmospheric pressure and a weight (W) of the collected second-stage expanded particles is measured so as to calculate the bulk density (=W/V).
On the other hand, the following method for measuring the bulk density of the pre-expanded particles that are easy to contract in a dry process at the time of pre-expansion is also disclosed (see Patent Document 3). That is, the pre-expanded particles are held in a sample collection vessel for a constant period of time before the dry process, and then, are subjected to the dry process, so that contraction of the pre-expanded particles is suppressed to minimum. Then, the bulk density of the pre-expanded particles is measured.
The following bulk-density measuring method when contracted pre-expanded particles are made to recover from contraction has been known (see Patent Document 4 and Patent Document 5). That is, a weight w of the pre-expanded particles is measured, the total amount of the pre-expanded particles is put into a measuring cylinder, and pressure in the measuring cylinder is reduced so as to make them recover from the contraction. Then, a volume v of the pre-expanded particles is measured by reading a scale of the measuring cylinder, and the bulk density of w/v is measured.